This invention relates generally to displays which have light emitting layers that are semiconductive polymers or oligomers (organic materials).
The different polymers and oligomers have been used for producing different displays. Light emitting oligomers are advantageous for producing smaller displays because of the process conditions (sputtering or vacuum evaporation) employed in fabricating oligomer displays. Oligomers have smaller chains, making them more stable materials than polymers because of their ability to withstand heat and moisture. Because of their relatively low solubility, oligomers are deposited by evaporation. This tends to make them most useful in smaller display applications.
Polymer displays, also known as organic light emitting displays (OLEDs) use layers of light emitting polymers. Unlike liquid crystal devices, the polymer displays actually emit light which may make them advantageous for many applications.
Generally, polymer displays use at least one semiconductive conjugated polymer sandwiched between a pair of contact layers. The contact layers produce an electric field which injects charge carriers into the polymer layer. When the charge carriers combine in the polymer layer, the charge carriers decay and emit radiation in the visible range.
One semiconductive conjugated polymer that may be used in polymer displays is poly(p-phenylenevinylene) (PPV) which emits green light. Another polymer which emits red-orange light is poly(methylethylhexyloxy-p-phenylenevinylene) (MEH-PPV).
Other polymers of this class are also capable of emitting blue light. In addition nitride substituted conjugated polymers may be used in forming the polymer displays.
Polymer displays may be formed from a substrate that may be a glass or metal foil substrate covered with an array of active elements. In one conventional structure the active elements may be thin film transistors (TFTs).
Conventionally, the polymer may be spun onto the substrate. However, lithography or photoprocessing of a conjugated polymer is a very difficult process. Electrodes are formed to control the light emission from the semiconductive polymer and then these polymers are spun onto the substrate. The polymer may include relatively small molecules like those used in fabric dyes, for example, having molecular weights between 500 and 1000. Another group of organics which are used with spin on resist are polymers with molecular weights between 100,000 and 1,000,000.
In addition to using spin on techniques, Cambridge Display, Ltd. reportedly has worked on a way to manufacture multicolored OLEDs deposited using ink-jet printing techniques. The promise of the technique is for large high resolution, low cost displays. The process uses a glass substrate with alternating stripes and grooves or channels and wells. In the prototype, the grooves form the subpixel areas into which various light emitting polymers are deposited. Ink-jet printing deposits red and yellow/green subpixels using PPV for green/yellow and rhodamine 101-doped PPV for red. The blue pixels are spin-coated, using a poly(dialkylfluorene) polymer. Surface treatments are utilized to realize fine patterning on either spin coat solutions or ink droplets with diameters of about 30 microns.
While the use of ink jet printing techniques for forming polymers displays has found some acceptance in forming displays with larger feature sizes, the technique has so far not been acceptable for forming smaller sized displays. In addition, the ink jet printing process is relatively limited in the resolution achievable.
With application techniques, such as ink jet printing techniques, the polymer display material is generally susceptible to air and moisture infiltration. In addition, the potential techniques for forming polymer displays are limited by the temperature sensitivity of the conjugated polymer material.
Producing a multi-color organic display is not an easy task. The available techniques for depositing different color layers are very difficult and also polymers which produce different colors have different lifetime and aging processes.
Thus, there is a continuing need for better ways to make polymer displays more cost effectively of smaller size.